Thanks for your quick response.
Perhaps it would be better to explain my situation:
I regularly have multiple browser windows open (usually 3 - 4) and each window has about 20 - 30 open tabs.
Accumulated, this takes up about 12GB of my 16GB available ram. I'm having trouble running anything outside my browser these days.

The browser windows are of course not always active, but contain valuable information about the things I'm working on. It would be great if I could "save" a window to the disk. This could save all the pages (and very importantly), the position on the page where the browser was open. This could free up about 4 GB per window.

Another great way would be to "save" the RAM memory of a window to disk if the window is not used for some time. I have a window open for example that I only need once a week, but it has all the tabs open where I need them.

In my perfect world, an application as a browser should never have to exceed 4 GB of ram.

@remisan, one thing that would help is to activate 'Advanced settings' (Preferences -> Browser -> tick 'Show Advanced Settings' at the bottom) and then turn on background loading (Preferences -> Browser -> tick 'Delay loading of background tabs'). Then, after you restart Opera, only the active tab will be loaded, and other tabs will be loaded when you navigate to them.